There are presently in use a variety of mass transport vehicles which are commonly acknowledged as being energy efficient and relatively inexpensive to operate and maintain. These mass transport vehicles can be considered to fall within two general categories, namely, trains and buses. Trains include long distance passenger trains as well as commuter trains and subways. Buses include such vehicles as executive transport vehicles, school buses, intra- or inter-city buses, vehicles used at airports or train terminals for transporting travelers from one terminal to another or for transporting travelers from a remote part of an airport to a passenger terminal, vehicles used to transport travelers from an airport or train terminal to motels and hotels, vehicles used to transport travelers from a city terminal to a remote airport, and vehicles used at airports for transporting passengers to auto rental facilities located at a distance from the terminal.
The most obvious common feature of all mass transport vehicles, regardless of type or size, is that they include a body enclosing the passenger carrying compartment and means for entering and leaving the vehicle, the last mentioned means being one or more doorways. The problems encountered with the design and operation of the doors in mass transit vehicles are common to all types of vehicles, whether trains or buses, whether small or large. For ease in the following description, reference will be made to buses in particular although it will be immediately recognized by one skilled in the art that such reference to buses is illustrative only and is equally applicable to any suitable type of mass transit vehicle.
In designing such buses, consideration must be given to the fact that the passengers may be entering and leaving the bus with hand luggage and a particularly wide door is necessary. Similarly, the passenger carrying hand luggage will be somewhat awkward in entering and leaving the bus and safety factors, therefore, must be considered. Most of the buses in common use utilize a so-called bi-fold door which is essentially a door having two panels which are hingedly joined to each other as well as being hingedly mounted on the body of the bus so that when the door is opened, it folds back upon itself. Some buses utilize a so-called double bi-fold door which is merely two bi-fold doors opening to opposite sides of the door opening.
There are several disadvantages associated with bi-fold doors in buses of this type. The main disadvantage is the tendency of the doors to stick due to the fact that the bodies of the vehicles have a variety of stresses applied to them as they travel on roughly paved surfaces and as the weight distribution shifts when passengers enter and leave the vehicle. These stresses and the shifting weight cause the body of the vehicle to flex and, thereby, cause misalignment of the doors with the tracks and associated mechanism.